Method and apparatus for removal of minute particles from a surface using thermophoresis to prevent particle redeposition

ABSTRACT

A method and apparatus for removing minute particles from a surface of a sample are provided that prevent redeposition of the particles onto the surface. By combining thermophoresis with laser assisted particle removal (LAPR), the methods and apparatus remove minute particles (for example, micrometer and nanometer sizes) and assure that they will not redeposit.

This application claims priority to Provisional Application No.60/220,418 filed on Jul. 24, 2000, the entire contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for removingminute particles from a surface. More particularly, the inventionrelates to a method and apparatus for removing minute particles from asurface using thermophoresis to prevent particle redeposition.

2. Background of the Related Art

Particle contamination of surfaces is a concern in many areas oftechnology. Two areas where such contamination can be a very significantproblem are optics, particularly those with critical optical surfaces,and electronic device fabrication. The effect of contaminants oncritical optical surfaces (coated or uncoated, dielectric or metal), forexample in high power laser optics, can lead to increased opticalabsorption and a decreased laser damage threshold. As minute particlescontaminate optical surfaces, they can serve as sinks for optical powerincident on the optical surfaces and thus produce localized heating andpossible damage. Large telescope mirrors, and space optics are otherapplications which require highly decontaminated critical opticalsurfaces.

In the electronics industry, particle contamination is an importantfactor in the manufacture of high density integrated circuits. Even inrelatively conventional technology using micron or larger circuitpatterns, submicron size particle contamination can be a problem. Todaythe technology is progressing into submicron pattern sizes, and particlecontamination is even more of a problem. For device fabrication,particles serve as “killer defects” for only the device that is particlecontaminated. The term “device” includes electronic devices, includingmasks/reticles, optical devices, medical devices, and other deviceswhere particle removal could be advantageous. A particle contaminatedmask/reticle prints every device with a defect. At the shorterwavelengths being developed for the next generation of lithography,materials for a protective pellicle for the mask are not available,making particle removal techniques an essential technology in thefuture. Contaminant particles larger than roughly 10% of the patternsize can create damage, such as pinholes, which interfere withfabrication processes (such as etching, deposition and the like), anddefects of that size are a sufficiently significant proportion of theoverall pattern size to result in rejected devices and reduced yield. Asan example, it has been found that the minimum particle size which mustbe removed in order to achieve adequate yield in a one Megabit chip(which has a pattern size of one micron) is about 0.1 microns.

Filtration (of air and liquid), particle detection, and contaminantremoval are known techniques used in contamination control technology inorder to address the problems outlined above. For example, semiconductorfabrication is often conducted in clean rooms in which the air is highlyfiltered, the rooms are positively pressurized, and the personnelallowed into the room are decontaminated and specially garbed beforeentry is allowed. In spite of that, the manufactured devices can becomecontaminated, not only by contaminants carried in the air, but also bycontaminants created by the processes used to fabricate the devices.

Removal techniques for contaminants should provide sufficient drivingforce for removal but without destroying the substrate. Moreover,acceptable removal techniques should provide a minimum level ofcleanliness in a reliable fashion. As the particle size decreases, theparticle weight becomes less significant as compared to other adhesiveforces binding the particle to the surface which it contaminates.Removal of such small particles can potentially damage the substrate.

In general, it has been found that submicron particles are the mostdifficult to remove. Many of the processes developed to clean integratedcircuits, such as ultrasonic agitation, are not effective for micron andsubmicron particles and indeed, sometimes add contaminants to thesubstrate.

Laser assisted particle removal has been described in U.S. Pat. No.4,987,286 issued to Susan D. Allen on Jan. 22, 1991, which is herebyincorporated by reference. U.S. Pat. No. 4,987,286 discloses a methodand apparatus for removing minute particles from a surface to which theyare adhered using laser technology, and further teaches the use of anenergy transfer medium to effect efficient laser assisted particleremoval (LAPR). As shown in FIG. 1, a condensed liquid or solid energytransfer medium 23, such as water, is interposed under and around acontaminant particle 22 to be removed from a substrate 20 to which theparticle is adhered. Thereafter, the medium 23 is irradiated using laserenergy 25 at a wavelength which is strongly absorbed by the medium 23causing explosive evaporation of the medium 23 with sufficient force toremove the particle 22 from the surface of the substrate.

Another particle removal technique has been to direct the laser energyinto the substrate. The laser heated substrate then transfers energyinto the energy transfer medium via conduction causing explosiveevaporation sufficient to remove the particle from the surface of thesubstrate. The laser energy can also be directed into the particle(s) tobe removed.

Both direct absorption by the energy transfer medium, and substrateand/or particle(s) absorption with subsequent heating of the energytransfer medium can result in efficient LAPR. However, advances intechnology have decreased the critical dimensions of various devices,such as, for example, magnetic hard drives, semiconductor devices, masksto make semiconductor devices, etc., and have also increased the surfacequality specifications for devices such as large telescope mirrors,space optics, high power laser optics, etc. Therefore, the ability toremove particulate contamination in a noncontact clean fashion becomesever more important.

One of the challenges of LAPR and other particle removal methods iskeeping the particles from redepositing on the surfaces, particularlyfor very small particles that are not significantly affected by gravity.Several options are available for preventing removed minute particlesfrom redepositing on the cleaned surface. For example, when particlesare removed in a vacuum, the mean free path of the particle is longenough to keep it from redepositing and a cooled surface can serve as aparticle trap. Also, gas jets parallel to the surface can be used toentrain particles and transport them away from the critical surface.

The above references are incorporated by reference herein whereappropriate for appropriate teachings of additional or alternativedetails, features and/or technical background.

SUMMARY OF THE INVENTION

An object of the invention is to solve at least the above problemsand/or disadvantages and to provide at least the advantages describedhereinafter.

The invention provides a novel method and apparatus for removing minute(for example, micrometer and nanometer size) particles from a surface,and preventing their redeposition. By combining thermophoresis withlaser assisted particle removal (LAPR), the present method and apparatusremoves minute particles and assures that they will not redeposit.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 is a diagram schematically illustrating a contaminated surfacewith adhered particles illustrating the practice of laser assistedparticle removal;

FIG. 2A is a diagram schematically illustrating a surface bearing acontaminant particle prior to introduction of an energy transfer mediumthereon;

FIG. 2B is a diagram schematically illustrating the introduction oflaser energy onto the contaminant particle;

FIG. 2C is a diagram schematically illustrating the removal of thecontaminant particle from the surface;

FIGS. 3A-3C schematically illustrate three exemplary ways in which theinvention can be implemented;

FIG. 4 is a schematic diagram of a system for performing the methodsaccording to the present invention; and

FIGS. 5-6 are schematic drawings of a particle gun according to theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Co-pending U.S. application Ser. No. 09909993 , which is herebyincorporated by reference, discusses and proposes methods and apparatusthat efficiently remove minute particles from the surface of asubstrate. The methods and apparatus according to the invention ofCo-pending U.S. application Ser. No. 09909993 overcome the shortcomingsof the prior art. However, as discussed above, one of the challenges ofLAPR and other particle removal methods is keeping the particles fromredepositing on the surfaces, particularly for very small (for example,micrometer and nanometer size) particles that are not significantlyaffected by gravity.

As previously discussed, FIG. 1 shows, in cross-section, a portion of asubstrate 20 bearing contaminant particles 22 which are adhered to asurface 21. The particles 22 are bound to the surface 21 by any of anumber of forces. The particles are deposited usually by a complexprocess which may include diffusion, sedimentation, inertia, andelectrical or electrostatic attraction. When the particles are verysmall, gravity is a minor source of adhesion, and other sources ofgreater significance are Van der Waals forces, electrostatic forces,capillary forces, and the like. Adhesion forces and the factorsnecessary for dislodging particles held by such forces will beconsidered in greater detail below. As the particles become smaller, theadhesion force per particle contact surface area increases rapidly, andremoval of such particles becomes a rather significant problem.

An energy transfer medium (ETM) may be interposed under and around theparticles 22, such medium being illustrated in the drawing as layer 23,which occupies interstices 24 formed between the adhered particles 22and the surface 21. FIGS. 2A-2B illustrates the introduction of an ETMonto a surface bearing a contaminant particle.

After preparing the surface for cleansing, energy is impinged upon thesurface to be cleaned. The energy may be targeted into, that is, at awavelength which is absorbed by, the particle, the substrate, or theETM, or some combination thereof.

In the example of FIG. 1, a laser beam 25 is directed at the surface 21,which carries the contaminant particles, and the interposed layer 24. Aquantity of energy is absorbed in the ETM, either directly or from thelaser heated particle or substrate, which is sufficient to causeexplosive evaporation on the medium. The quantity of material interposedunder and around the particle is such that, when explosive evaporationoccurs, the particle is driven from the surface by the force of theexplosion, as shown in FIG. 2C. In effect, the laser energy incident onthe surface is converted by the ETM from potential to kinetic energy,and is transferred to the particle, driving it from the surface to whichit had been adhered.

The methods and apparatus according to the invention use thermophoresisto prevent the redeposition of dislodged particles onto the surface of asample or substrate. It has been known that a temperature gradient in agas causes small particles suspended in the gas to migrate in thedirection of decreasing temperature. This phenomenon is calledthermophoresis. The methods and apparatus according to the inventioncombine thermophoresis with laser assisted particle removal (LAPR) toremove difficult to remove particles from a surface of a sample and toprevent their redeposition.

Thermophoresis was discovered in the steel industry in 1910. A thermalgradient produces a net force on a particle small enough to exhibitBrownian motion toward the colder side of the gradient. This forceexists because the hotter gaseous molecules near the surface that isbeing protected have more kinetic energy to impart to the smallparticle, tending to force it toward the colder part of the gas.

It has been demonstrated by Lenny Klebanoff, Dan Radar, and DanielDedrick at Sandia National Laboratories that a temperature gradient ofapproximately 15 K/cm will prevent approximately 0.2 μm polystyreneparticles flowing from a “showerhead” from depositing on a mask surface.This temperature gradient can be produced, for example, by cooling aplate above the surface to be protected, by heating the surface to beprotected, or by some combination thereof. Pressures as low asapproximately 30 mT can be utilized to create a thermophoretic force.

Experiments were also conducted by Klebanoff et al. for approximately 10K/cm temperature gradients in which a test wafer, protected bythermophoresis, was exposed to class ˜7 million Ar gas at approximately1 Torr pressure and laden with approximately 0.24 micron diameterparticles. Post exposure scans of the wafer indicated particleprotection factors in excess of ˜10⁶. The model used by Klebanoff et al.indicated that the technique would work down to approximately 50 nmparticles.

Obviously, there is no thermophoresis in vacuum, but there are manyprocesses with particle surface contamination problems that operate in agaseous environment at atmospheric, low or high pressure. The readilyexecutable redeposition prevention process for use with LAPR accordingto the invention in atmosphere would be advantageous for many processes,including but not limited to cleaning semiconductor wafers and masks,cleaning high resolution optics such as large telescope mirrors,cleaning critical surfaces in space, cleaning high power laser optics,etc.

FIGS. 3A-3C schematically illustrate three exemplary ways in which theinvention can be implemented. In FIG. 3A, a temperature control unit 98is provided which controls the temperature of a plate 98A on which asample or substrate 20 is placed. In FIG. 3B, a plate 99A andcorresponding temperature control unit 99 are disposed above, or at apredetermined distance D from a sample or substrate 20. FIG. 3Cillustrates an embodiment, which is a combination of the embodiments ofFIGS. 3A-3B. By controlling the temperature control units 98 and/or 99,a temperature gradient can be created which will draw dislodgedcontaminant particles away from the surface of a sample or substrate 20and ensure that they do not redeposit.

Further, temperature control units, such as those shown in FIGS. 3A-3B,can also be used to create a “particle gun”. That is, the temperaturecontrol units could be manipulated to control the velocity and directionof particle flow. The velocity of the particles would be dependent onthe temperature gradient as well as the size of the respective particlesbeing manipulated. Such a particle gun concept could be used toaccelerate particles toward a desired target.

FIG. 4 shows an apparatus configured for practice according to oneembodiment of the invention. The apparatus includes a chamber 50.Mounted on a support (not shown) in the chamber 50 is a substrate 54 tobe cleaned. The substrate 54 has a surface 55 which contains contaminantparticles (not shown in the scale of FIG. 4) which are to be removed.

For the purpose of controlling the adsorption and the description ofliquid materials such as water, a cooling source 56 is coupled byconduit 57 to the substrate 54. As noted above, the temperature of thesubstrate 54 may be reduced to enhance water absorption to the surface55.

An ETM can be applied as a liquid or gas. For the purpose of dosing thesurface with a liquid ETM, for example, water or an alcohol/watermixture, a liquid source 60 is provided and is coupled by a dosing tube61 to the surface 55 of the substrate 54. Liquid supplied by source 60travels through the dosing tube 61 and is applied to the surface 55 atthe appropriate temperature to assure adsorption on the surface and ininterstices under and around the contaminant particles. After waterdosing, the temperature of the substrate 54 can be maintained by thecooling source 56, such that adsorption of surface water occurs whilemaintaining water in the interstices under and around the contaminantparticles and the surface.

A plate 99A and a corresponding temperature control unit 99 are providedat a predetermined distance from the substrate 54 to create atemperature gradient according to this embodiment of the invention. Alaser source 64 is provided with means 66 for steering a laser beam 65,if necessary. Additional beam guiding means can be provided to guide thelaser energy to the substrate despite obstacles.

After a sample is prepared for cleaning, the laser source 64 isenergized, and outputs pulses of energy in a beam illustrated at 65 tothe surface 55. As an alternative, the sample itself can be moved withinthe chamber 50 to direct the laser beam to the desired area of thesurface 55. In any event, the beam 65 is focused on areas of the surface55 to be cleaned and the laser 64 pulsed to couple adequate energy tothe surface 55.

As seen in FIG. 4, the sample 54 is mounted such that particles whichare driven from the surface 55 can fall gravity assisted withoutredepositing on the surface. Additionally, the temperature control unit99 creates a temperature gradient that ensures that the particles do notredeposit on the surface of the sample 54.

The present invention can also be used to form a particle gun, asmentioned briefly above, such as that shown in FIGS. 5-6, which woulddeposit particles onto a target substrate. This can be useful in themanufacture of, for example, computer monitors. Particles interposedbetween a mask and a polymer, during imprinting of a polymer baseddiode, will create rows of pillars, creating a photonic bandgapmaterial. See “Dusty Lab May Revolutionize LEDs,” Photonics TechnologyWorld, September 2000, which is hereby incorporated by reference. Finecontrol of the height and distribution of the pillars allows control ofcolors emitted by an LED, which are determined by microcavities in thepolymer. See id. Instead of manufacturing each color with differentlight-emitting materials, the entire range of color can be produced withone material by controlling the height and distribution of the pillars.See id.

The particle gun according to the invention, discussed above and shownin FIGS. 5-6, can be used to deposit particles on a substrate in apredetermined pattern and/or in layers. For example, transparent tapecan be used with different kinds, sizes, etc. of particles disposed onthe tape at different portions thereof. The tape can then be moved intothe path of the laser energy to expose different portions of the tape tothe laser energy.

The particle gun 100 in FIG. 5 includes a substrate 120, and an energytransfer medium 123 with particles 122 deposited thereon. Laser energy125 provided by a laser (not shown) is directed at the substrate/ETMcombination. The particles 122 are accelerated from the surface of thesubstrate 120 towards a target substrate 140, upon which the particles122 adhere as shown in FIG. 6. The temperatures of the substrate 120 andthe target substrate 140 can be altered to affect particle depositiondensity and particle deposition distribution patterns. For example, thesubstrate 120 can be cold relative to a warm target substrate 140,preventing ETM redeposition on the target substrate 140 resulting in dryparticle deposition.

The foregoing embodiments and advantages are merely exemplary and arenot to be construed as limiting the present invention. The presentteaching can be readily applied to other types of apparatuses. Thedescription of the present invention is intended to be illustrative, andnot to limit the scope of the claims. Many alternatives, modifications,and variations will be apparent to those skilled in the art. In theclaims, means-plus-function clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents but also equivalent structures.

What is claimed is:
 1. A method of removing one or more particle(s)adhered to a surface of a sample, comprising: arranging an energytransfer medium under and around the one or more particle(s);irradiating the one or more particle(s), the surface and/or the energytransfer medium with laser energy from a laser energy source; absorbingsufficient energy in the one or more particle(s), the surface, thesubstrate, and/or the energy transfer medium to dislodge the one or moreparticle(s); and creating a temperature gradient adjacent to the surfaceusing a heating and/or cooling source separate from the laser energysource to prevent the one or more particle(s) from redepositing on thesurface.
 2. The method according to claim 1, wherein said irradiatingstep comprises irradiating the energy transfer medium with laser energy,and said absorbing step comprises absorbing sufficient energy in theenergy transfer medium to cause explosive evaporation thereof withsufficient force to dislodge the one or more particle(s).
 3. The methodaccording to claim 1, wherein said step of creating a temperaturegradient adjacent to the surface to prevent the one or more particle(s)from redepositing on the surface comprises heating the sample.
 4. Themethod according to claim 1, wherein said step of creating a temperaturegradient adjacent to the surface to prevent the one or more particle(s)from redepositing on the surface comprises cooling a plate disposedadjacent to the surface of the sample.
 5. The method according to claim1, wherein said step of creating a temperature gradient adjacent to thesurface to prevent the one or more particle(s) from redepositing on thesurface comprises both heating the sample and cooling a plate disposedadjacent to the surface of the sample.
 6. The method according to claim2, wherein the laser energy is sufficient to be absorbed by the energytransfer medium, either directly or by conduction from the substrate. 7.The method according to claim 2, wherein the energy transfer medium isat least one of a uniform layer of thickness, absorbed into intersticesunder and around the one or more particle(s) to be removed, and acombination thereof.
 8. The method according to claim 7, wherein theenergy transfer medium is a uniform layer of thickness.
 9. The methodaccording to claim 7, wherein the energy transfer medium is absorbedinto interstices under and around the one or more particle(s) to beremoved.
 10. A method of removing one or more particle(s) adhered to asurface of a sample, comprising: irradiating the one or moreparticle(s)/sample combination with laser energy from a laser energysource; absorbing sufficient energy in the one or moreparticle(s)/sample combination to dislodge the one or more particle(s);and creating a temperature gradient adjacent to the surface using aheating and/or cooling source separate from the laser energy source toprevent the one or more particle(s) from redepositing on the surface.11. The method according to claim 10, wherein said step of creating atemperature gradient adjacent to the surface to prevent the one or moreparticle(s) from redepositing on the surface comprises heating thesample.
 12. The method according to claim 10, wherein said step ofcreating a temperature gradient adjacent to the surface to prevent theone or more particle(s) from redepositing on the surface comprisescooling a plate disposed adjacent to the surface of the sample.
 13. Themethod according to claim 10, wherein said step of creating atemperature gradient adjacent to the surface to prevent the one or moreparticle(s) from redepositing on the surface comprises both heating thesample and cooling a plate disposed adjacent to the surface of thesample.